1. Field of the Invention
The present invention relates to a rotation angle sensor that is able to detect a rotation angle of a steering shaft.
2. Description of the Related Art
In the related art, there has been known a rotation angle sensor installed on a vehicle for detecting a rotation angle of a steering shaft.
FIG. 1 is a schematic view of a rotation angle sensor 100 of the related art. As shown in FIG. 1, the rotation angle sensor 100 is comprised of a main gear 101, detection gears 102,103, magnet sensors 104,105 and a processor 106.
The main gear 101 integrally rotates with a steering shaft The detection gears 102, 103 rotate directly with the main gear 101 at a higher speed than the main gear 101. The magnet sensor 104 detects an absolute angle of the detection gear 102 in a value ranging from 0 to 180 [deg] to output a detection signal. The magnet sensor 105 detects an absolute angle of the detection gear 103 in a value ranging from 0 to 180 [deg] to output a detection signal. The processor 106 calculates an absolute angle of the steering shaft depending on detection signals delivered from the magnet sensors 104, 105. As used herein, the term “absolute angle” refers to a rotation angle uniquely determined within a rotatable range. For instance, if an apparent rotation angle of the steering shaft lies at 10 [deg], the absolute angle of the steering shaft lies at 10 [deg], 370 [deg], 730 [deg], . . . depending on the number of rotations of the steering shaft.
Further, the processor 106 monitors cycle variations of the magnet sensors 104, 105 and calculates a given numeric value k based on the detection signals delivered from the magnet sensors 104, 105 and gear teeth of the detection gears 102, 103. Then, the processor 106 discriminates based on the calculated numeric value k and the cycle variations of the magnet sensors 104, 105 to find whether the absolute angle of the steering shaft, calculated by the processor 106, falls in an abnormal value.
More particularly, the processor 106 monitors the cycle variations of the magnet sensors 104, 105 and, as a result, if the numeric value k drastically varies due to the cyclic variations to an extent that is normally unthinkable or when the numeric value varies with no cycle variations, discriminates that the calculated absolute angle of the steering shaft lies in the abnormal value.
Also, as used herein the term “a cycle of the magnet sensor 104” refers to an angle at which the steering shaft rotates during a period in which the detection gear 102 rotates at an angle of 180 [deg] and the term “a cycle of the magnet sensor 105” refers to an angle at which the steering shaft rotates during a period in which the detection gear 103 rotates one turn.
However, under circumstances where the gear teeth of the detection gears 102, 103 take the other value than particular gear teeth, the above described technology encounters issues wherein the processor 106 is hard to discriminate whether the absolute angle of the steering shaft calculated by the processor 106 falls in the abnormal value.
Further, if the steering shaft has to remain operative, the processor 106 cannot discriminate whether the absolute angle of the steering shaft, calculated by the processor 106, lies at the abnormal value. Accordingly, when the rotation angle sensor 100 is powered on, the processor 106 cannot discriminate whether the absolute angle of the steering shaft, calculated by the processor 106, falls in the abnormal value.